The benefits of automobiles have unfortunately come with tragic costs. There were 37,261 fatalities on American roads in 2008. Automobile collisions rank as the eleventh leading cause of death irrespective of age and the first leading cause of death for those between 15 and 24 years old. Preventing such collisions would protect property, health, and most importantly lives. Recent maturation of some fundamental technologies has allowed engineers to develop new means of preventing automobile collisions.
These new collision avoidance technologies may rely upon communication, either in part or entirely, as a means of co-operatively avoiding collisions. Collaborative efforts between automotive Original Equipment Manufacturers (OEMs) have demonstrated the feasibility of vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communication as means of co-operatively avoiding collisions.
Wireless communication in such systems allows vehicle position, velocity, and other relevant parameters to be shared among neighboring vehicles, which in turn allows for calculating collision risks, providing warnings to the driver, and potentially even initiating mitigating actions. To be effective, these co-operative vehicle safety technologies require that all involved vehicles use the same communication protocol. The draft standard known as Dedicated Short Range Communications (DSRC) defines a communication medium and a message set for such a communication protocol, thereby providing for the communication and sharing of various parameters among neighboring vehicles.
Many important parameters usually are already available within proprietary in-vehicle networks. Parameters such as brake status and turn-signal status may be used to infer a driver's intentions and need only be converted into standard packaging for broadcasting to other vehicles. Arguably the most critical pieces of shared information are vehicle position and time. For co-operative safety applications to work, position and time need to be expressed in relation to other vehicles and thus require a common reference frame.
In this regard, the use of GNSS (Global Navigation Satellite Systems) has proven valuable. A GNSS includes a constellation of multiple man-made Earth-orbiting satellites with precisely determinable orbits. A user of the system employs a device (a GNSS receiver) to obtain positioning information in Earth-based absolute co-ordinates of latitude, longitude, and height, by solving equations involving measurements of signal travel times from at least four GNSS satellites. The first such system, and the one most widely used, is the Global Positioning System (GPS) created by the United States government. Although the term GNSS includes GPS, it is a generic term and encompasses any such system, including for example GLONASS (a Russian GNSS) and Galileo (a European GNSS).
GPS has nearly global coverage and accessibility in open sky conditions. However, the availability of GNSS-based positioning information in obstructed environments—including, significantly, areas like dense urban areas—is often poor. Efforts are under way to improve the performance of GNSS-based positioning systems in obstructed environments by combining GNSS sensors with other sensors. In order to be evaluated, these improved GNSS-based positioning systems share a need to calculate a positioning error as a difference between a reference trajectory and a trajectory measured by the improved GNSS-based positioning system, although the systems may differ in how the reference trajectory is determined.
Reference trajectories may be determined, for example, through GNSS-based methods incorporating high-grade inertial sensors or differential corrections. However, such methods may become ineffective for extremely difficult environments, or may become prohibitively expensive for projects of limited budget. Various other methods of determining reference trajectories may have other drawbacks that may make them impracticable due to a dependence upon GNSS-derived positioning information. Accordingly, there is a need for a method of testing improved GNSS-based positioning systems that does not require the use of GNSS in determining reference trajectories.